Gynecomastia

The growth of male breast tissue. Despite the alarming name (Greek gyne “woman” + mastos “breast”), it is a normal transient finding in roughly half of all boys during puberty, and a normal finding again in most newborn boys. Pathological gynecomastia exists, but it is the exception — the page below explains the physiology first, then the conditions that can drive it pathologically.

The ratio principle

The male breast bud is the same tissue as the female one: a small disc of estrogen-responsive duct epithelium under the nipple. Estrogen makes it proliferate; androgens oppose that proliferation. The breast therefore responds not to the absolute level of either hormone but to the estrogen-to-androgen ratio acting on it.

Anything that tips that ratio toward estrogen — more estrogen, less androgen, more aromatisation, more bioavailable estrogen relative to bioavailable testosterone — can produce gynecomastia. That single principle organises every cause below.

A separate distinction matters before any workup:

• True gynecomastia — firm, rubbery glandular tissue concentrically arranged behind the nipple. Hormonal.
• Pseudogynecomastia — soft adipose tissue in an overweight boy, with no glandular component. Not hormonal; not on this page.

A pinch test distinguishes the two: true glandular tissue is felt as a discrete disc; fat slides under the fingers without a defined edge.

The dominant cause in boys: pubertal gynecomastia

By a wide margin, this is the most common form. About half of all boys develop palpable breast tissue at some point during Tanner G2–G4 (typically ages 10–14, peaking around G3).

Why it happens:

• Early in puberty, estradiol rises before testosterone catches up. Adrenal androgens (from adrenarche, which begins a few years earlier) are aromatised to estrogens in peripheral fat and skin while the testes have only just begun to ramp up testosterone production.
• The estrogen-to-androgen ratio is briefly elevated, and the breast bud responds by proliferating.

What it looks like:

• A firm, sometimes tender, rubbery disc 1–3 cm across, directly under the nipple.
• Often asymmetric — one side first, or only one side.
• Usually develops over months, not weeks.

Course:

• Self-limited in the great majority of cases. Resolution within 6–24 months, as testosterone rises during G4 and the ratio normalises.
• About 90% of boys with pubertal gynecomastia have it fully resolve by the end of puberty.
• A small minority (perhaps 5–10%) have persistent pubertal gynecomastia that does not regress — the breast tissue eventually fibroses and stops responding to hormonal correction, leaving a permanent soft-tissue change.

Management:

• Reassurance and observation for typical, transient cases — this is the most common and correct intervention.
• Watchful waiting through one or two puberty checks; document size, side, and tenderness.
• Pharmacological treatment (selective estrogen-receptor modulators, aromatase inhibitors) and surgical excision exist for severe or persistent cases, but are reserved for situations causing significant distress or failing to resolve.

Other physiological forms

• Neonatal gynecomastia — present in 60–90% of newborn boys, from transplacental maternal estrogens. Resolves within a few weeks as maternal estrogens clear. Sometimes accompanied by witch’s milk — a transient discharge from the nipple, also estrogen-driven.
• Refeeding gynecomastia — when nutrition is restored after a long period of severe malnutrition, the HPG axis switches back on like a second puberty, and a transient gynecomastia can follow. Seen historically in prisoner-of-war camps and now in recovery from severe restrictive eating disorders.
• Aging gynecomastia — declining testosterone and rising peripheral aromatase activity in adipose tissue raise the ratio in late life; outside the scope of male development but listed for completeness.

Pathological causes

When gynecomastia is unusual — appearing outside the expected pubertal window, persistent past G5, very large, rapidly progressive, or unilateral with hard or fixed tissue — pathology is in play. Organised by which side of the ratio the cause tips:

Drug-induced (the single largest non-physiological category)

A long list, broken out by mechanism:

• Adds estrogenic activity
  • Anabolic steroids — aromatise to estrogens at the doses used by athletes; also cause withdrawal hypogonadism.
  • Exogenous estrogen exposure — lavender and tea tree oil products (well-documented in prepubertal boys), industrial estrogens, accidental contact with hormone-replacement creams.
  • Marijuana, alcohol, opioids — varying mechanisms.
• Blocks androgen action
  • Spironolactone — diuretic that is also an androgen-receptor antagonist; classic offender, often used to treat acne.
  • Cimetidine, ranitidine — H2 antacids; block androgen receptors.
  • Finasteride — 5α-reductase inhibitor (acne, hair loss).
  • Ketoconazole — antifungal that blocks androgen synthesis.
• Suppresses the HPG axis (lowering testosterone)
  • Dopamine antagonists — antipsychotics (risperidone, haloperidol), the antiemetic metoclopramide. Act through hyperprolactinemia (see prolactin axis) and the HPG inputs.
  • Opioids — directly suppress GnRH pulsatility.
• Damages testicular tissue
  • Chemotherapy, radiation, some antiretrovirals (efavirenz in particular).

A careful medication history is one of the first steps in the workup of any unexpected adolescent gynecomastia.

Hormonal disorders

• Klinefelter syndrome (47,XXY) — the most common congenital cause of pathological gynecomastia. About half of Klinefelter boys develop gynecomastia at puberty, because the extra X impairs testicular development → low testosterone → relative estrogen excess. Also raises male breast cancer risk substantially. Suspect in a pubertal boy with persistent or large gynecomastia, tall stature, small firm testes (often < 6 mL when other Tanner features are at G4), and learning differences. See the chromosomal-failure section of HPG deviations.
• Primary hypogonadism of any cause — cryptorchidism, testicular trauma, mumps orchitis, chemotherapy damage. Low androgen output tips the ratio toward estrogen.
• Hyperthyroidism — raises sex-hormone binding globulin (SHBG), which binds testosterone preferentially over estrogen, lowering the free fraction available to act on tissue.
• Hyperprolactinemia — prolactinoma, dopamine-antagonist drugs, primary hypothyroidism. Acts indirectly: prolactin suppresses GnRH → low testosterone → ratio tips. Galactorrhea (milk discharge from the nipple) is often a more characteristic finding than gynecomastia itself.
• Androgen insensitivity syndromes — genetic 46,XY individuals whose tissues cannot respond fully to androgens; estrogen acts unopposed. Range from complete (raised female) to partial (Reifenstein syndrome, with infertility and gynecomastia).
• Aromatase excess syndrome — rare autosomal-dominant overexpression of aromatase, which converts androgens to estrogens. Prepubertal gynecomastia plus short adult stature from early growth-plate closure.
• Congenital adrenal hyperplasia — abnormal adrenal steroid synthesis can shunt substrates into estrogen pathways.

Tumours (uncommon but important to recognise)

• hCG-secreting tumours — testicular germ-cell tumours (seminoma, choriocarcinoma in adolescents), occasionally hepatic or lung tumours. hCG mimics LH, overstimulating Leydig cells; both testosterone and (via more substrate for aromatase) estrogen rise, and estrogen often wins. A prepubertal boy with new gynecomastia plus premature pubic hair or testicular asymmetry on exam needs hCG checked and the testes scanned.
• Estrogen-secreting tumours — adrenocortical tumours, rare Leydig-cell or Sertoli-cell tumours of the testis. Suspect when gynecomastia is rapid-onset, unilateral, or accompanied by other signs of estrogen excess.
• Pituitary tumours — including prolactinomas; usually act through HPG suppression rather than direct gland growth.

Systemic illness

• Chronic liver disease — the liver clears estrogens; when it fails, estrogens rise. Rare in boys but seen in advanced metabolic or congenital liver disease.
• Chronic kidney disease — multifactorial: low testosterone, hyperprolactinemia, drug effects.

When to investigate — the red flags

Most pubertal gynecomastia needs no workup beyond a careful history and exam. The signs that suggest pathology rather than physiology:

• Wrong age — prepubertal (before any other Tanner features) or persisting well past G5.
• Wrong size — a disc > 4 cm or extending beyond the areolar margin.
• Wrong texture — hard, fixed, irregular, with skin changes or nipple discharge (raises the rare possibility of breast cancer, especially in Klinefelter syndrome).
• Wrong tempo — rapid progression over weeks rather than months.
• Wrong context — other endocrine signs (small testes for the Tanner stage, asymmetry, galactorrhea, visual-field defects, signs of hyperthyroidism).
• Drug exposure — a careful list of prescription, over-the-counter, supplement, and recreational substance use.

When investigation is warranted, the panel typically includes: serum testosterone, LH, FSH, estradiol, prolactin, TSH and free T4, hCG, liver and kidney function, and a karyotype if Klinefelter is suspected.

Key idea

Gynecomastia is a ratio problem, not a single-hormone problem. In boys it is overwhelmingly pubertal physiology — a transient mismatch between estrogen and testosterone during Tanner G2–G4 that resolves itself. The pathological causes worth screening for are the ones that break the ratio persistently: a drug, a chromosomal disorder (Klinefelter), an hCG-secreting tumour, or a hormonal disease that drops testosterone or raises estrogen. Reassurance is the right answer most of the time; the workup is for the cases that look wrong on history or exam, not for every breast bud in a 13-year-old.